This invention relates to the separation and purification of platinum and palladium with particular reference to the removal of these metals from associated metals such as base metals, and other metals of the platinum group.
In this specification the term "base metal" is used to mean any impurity other than a member of the platinum group and gold.
Platinum and Palladium generally occur in nature in ores which contain further platinum group metals, silver gold and base metals. Well known processes are used to produce a concentrate having a platinum group metal concentration of at least 5% by mass. Treatment of these concentrates usually includes the following consecutive basic steps:
1. The concentrate is leached in hydrochloric acid using chlorine or nitric acid to oxidise the Platinum group metals. This dissolves much of the base metal content, and most of the gold, silver, platinum and palladium, while a large proportion of the secondary platinum group metals (i.e. Rh, Rh, Ir and Os) and the more refractory Pt, Pd and Au fractions are left in the leach residue. Processes for recovery of the platinum group metal content of the residue are generally long and complicated.
2. A reductant is used to precipitate gold as the metal which is then removed from the leach liquor by filtration.
3. An impure salt, ammonium chloroplatinate, (NH.sub.4).sub.2 PtCl.sub.6, is then precipitated and removed by filtration. The palladium is subsequently precipitated, usually as the dichloro diammino complex Pd(NH.sub.3).sub.2 Cl.sub.2.
4. Both the platinum and palladium precipitates are impure and are purified by further operations including, in the case of platinum, the calcination of the impure salt to metal, and the dissolution thereof in an oxidising, acid medium prior to purification.
These purification steps involve further losses of Pt and Pd so that the single-pass recovery for both metals is quite low and extensive amounts of material must be recycled.
The problem of incomplete and non-dissolution of some of the platinum group metals has been solved to a large extent by the process disclosed in our U.S. application Ser. No. 652,283, now abandoned.
This process involves alloying the platinum group metal concentrate with aluminium and the subsequent formation of a leach liquor containing the majority of the platinum group metal content of the alloy.
Processes for the separation of the various metals from such a liquor and from each other are known but are generally inefficient and not sufficiently selective. In the case of Pt and Pd, a basis for an improved separation process lies in the greater tendency of these metals to form anionic chlorocomplexes than the associated base metals and other platinum group metals to form such complexes. Attempts have been made to exploit this difference in behaviour by using anion-exchange resins and solvents. Typical examples are the use of tertiary amines e.g. Alamine 310, (a General Mills Inc. product sold under the Trade Mark "Alamine"), and a very weak base solvent extractant such as tributyl-phosphate. These anion exchanges do indeed have fairly high selectivity but all have associated with them serious disadvantages which render their use on a large-scale impractical.
These difficulties are different in each case but can be summarised as follows:
a. The solvents or resins, while extracting Pt and Pd are difficult to strip in practice. This applies to extractants such as Alamine 310.
b. Where the solvents can be easily stripped e.g. tributyl-phosphate (TBP) their capacity and distribution coefficient on extraction are very low.
c. Solvents such as primary and secondary amines can also be used and their capacity and distribution coefficients are between that of TBP and those of tertiary amines. Compounds such as these, however, have a tendency to form complexes with the extracted metal by the following type of reaction:
(i) Extraction takes place according to the reaction: EQU 2R.sub.2 NH.sup.+ Cl.sup.- +PdCl.sub.4.sup.2- .fwdarw.(R.sub.2 NH.sup.+).sub.2 (PdCl.sub.4.sup.2-)+2Cl.sup.- PA1 (ii) Complex formation takes place according to the reaction: EQU (R.sub.2 NH.sup.+).sub.2 (PdCl.sub.4).sup.2- .fwdarw.Pd(R2N).sub.2 Cl.sub.2 +2HCl.
The ion pair formed in reaction (i) can be destroyed by stripping with, for example, a strong hydrochloric acid solution but the complex formed in reaction (ii) cannot be so destroyed. This means that the extractants become poisoned at a rate unacceptable for commercial use.
Added to these difficulties is the fact that while a large measure of purification from base matals and secondary platinum group metals is achieved, sufficient of these are coextracted to render further purification necessary.
It is thus an object of this invention to provide a solvent having the following characteristics:
1. A high capacity and distribution coefficient for both platinum and palladium under suitable loading conditions,
2. An ability to be stripped of platinum or palladium or both by simple means,
3. A sufficiently high selectivity for platinum or palladium or both so that the strip liquor is pure enough to allow production of either pure platinum or palladium or a mixture thereof after separation of these metals and,
4. A high resistance to complex formation whereby irreversible extraction occurs.